var posSup = (screen.availHeight/2) - 190;
var ex = navigator.appName == "Microsoft Internet Explorer" ? "si" : "no";
if (ex && navigator.userAgent.indexOf('Opera') >= 0) ex = "no";

function posicion(){
	if(window.pageYOffset){
		scrollUp =  window.pageYOffset;
	}
	else {
		scrollUp = Math.max(document.body.scrollTop,document.documentElement.scrollTop);
	}
	scrollUp += posSup;
	document.getElementById('frm_autopromo').style.top = scrollUp + "px";
}

function posiciona(){	
	var d = document.getElementById('frm_autopromo');
	d.style.top = posSup + "px";
	if(ex == "no"){
		d.style.position = 'fixed';
	}			
	else if (ex == "si"){
		d.style.position = 'absolute';
		onscroll = posicion;
	} 
}		
posiciona();